well, in theory, you can't have "too large" a sump ... the bigger the better - just remember to take into account your total
system water capacity when dosing, adding meds and such.
if you have the room for a 55 gallon under your display tank, then more power to you!
for the spacing between baffles, it really depends on your GPH
- you want to have enough room for the water to flow without being restricted... I would guestimate at least 1.5"
the microbubbles are caused more by a mal-adjusted skimmer and "waterfall" effect from a low water level - keeping your skimmer running at optimal configuration and keeping your water level high should go a long ways in reducing bubbles.
the baffles really serve to 'organize' the water into a more orderly flow, smoothing out the turbulence cause by the overflow drains.